Wednesday, April 23, 2014

Forensic Use of Color

Color-changing polymer maps fingerprints

Detecting perspiration pinpoints people’s pores

SWEAT PRINT Tiny pores on people’s fingertips ooze sweat droplets (shown red in fluorescence image) that can be detected with a new color-changing polymer. The technique could supplement traditional fingerprinting methods, which rely on impressions left by finger ridges.

Sweaty fingers make tidy prints. Beads of perspiration seeping from a person’s pores can leave detailed maps of the fingertips, and a new technique can detect the sweat.
Human finger pores ooze salty drops of water about the size of pinpricks, says materials scientist Jong-Man Kim of Hanyang University in Seoul, South Korea.
He and colleagues created color-changing polymers that snap from blue to red when they touch the tiny droplets. Individual polymer units look like teeny tadpoles, with bulbous heads and skinny tails. When packed tightly together, they form stacked sheets that appear blue. But when water swells the polymers’ heads, the crowded sheets twist apart and absorb shorter wavelengths of light, making the sheets look red.
Pressing a finger to a polymer-coated film instantly colored it with red dots, Kim’s team reports April 29 in Nature Communications. Kim thinks the polymers could improve existing fingerprinting technologies, which analyze impressions left by finger ridges’ loops, arches and whorls. Pores speckle these ridges, creating unique dot patterns that match up with traditional fingerprints.
Forensics teams can pick up 10-year-old dots of sweat left on a piece of paper even in the absence of fingerprints, Kim says, but the dot data are often tossed because no one had a simple way to map people’s pores.

IT’S A MATCH Fluorescence image of sweat pores (red) overlaid on a scanned image of a fingerprint reveal similar patterns in pores and finger ridges.